1. Field of the Invention
The present invention relates to a scanning electron microscope, and particularly to an improvement in a scanning electron microscope of the type wherein secondary electrons reflected from a specimen or sample by irradiation of primary electrons thereto are gas amplified by gas in a sample chamber.
2. Description of the Prior Arts
In a conventional scanning electron microscope, secondary electrons reflected from a specimen or sample placed within a vacuum are detected by light emission from a scintillator. In contrast, in a so-called environmental control type scanning electron microscope, secondary electrons reflected from a sample placed in a low-pressure gas such as water vapor or the like are gas magnified (electron magnified), and the gas magnified secondary electrons are directly detected by a detecting electrode. Such a scanning electron microscope of an environmental control type allows the observation of various specimens or samples such as those containing water, which cannot be observed by conventional electron microscopes. In a usual low acceleration scanning electron microscope, the quantity of primary electrons impinging on the sample is about the same as that of secondary electrons reflected from the sample. On the contrary, in the scanning electron microscope of an environmental control type, due to a relatively large acceleration voltage for an electron beam, the quantity of secondary electrons reflected from the sample tends to be smaller than that of a primary electron beam impinging on the sample. As a result, in the environmental control type electron microscope, the sample is apt to become negatively charged. The environmental control type electron microscope, therefore, needs to have means for neutralizing the negatively charged sample.
FIG. 3 is a view showing an example of a conventional scanning electron microscope of an environmental control type. A vacuum chamber 2 (in actuality, comprising a plurality of rooms which are separated from each other by partitions having an aperture, i.e. aperture plates, for differential exhaust) containing an electron gun 3 and a sample chamber 9 neighbor each other with a pressure limiting member or plate 7 having an opening or aperture 7a at the center thereof interposed therebetween. The pressure limiting apertured plate 7 is secured to a lens barrel 1 through an insulator 8. A condenser lens 4, an objective lens 6, and a deflector 5 are arranged between the electron gun 3 and the pressure limiting plate 7. Gas (e.g. water vapor) having an electron multiplying effect is supplied to the sample chamber 9 from a gas source, not shown. Pressure of the gas in the sample chamber 9 is held at about 0.1 Torr to several tens of Torr by a vacuum pump 12. The gas in the sample chamber 9 flows into the vacuum chamber 2 through the aperture 7a in the pressure limiting plate 7. However, a vacuum pump 11 maintains the vacuum chamber 2 at a lower pressure (in the state of a higher degree of vacuum) than the inside of the sample chamber 9. For example, the gas pressure of the vacuum chamber 2 is held at about 10.sup.-2 to 10.sup.-3 Torr at a position just above the pressure limiting plate 7. In actuality, the vacuum chamber 2 is divided into a plurality of rooms by members or plates defining an aperture or opening. Each room is provided with a vacuum pump, and the electron gun 3 is disposed in a room having the highest degree of vacuum. A specimen 10 or sample formed of insulator, i.e. an object of observation, is contained within the sample chamber 9.
According to the prior art, the pressure limiting plate 7 also serves as a secondary electron detector. A positive voltage relative to the sample 10 is applied to the pressure limiting plate 7 by a variable voltage source 13. A secondary electron signal obtained from the pressure limiting plate 7 is fed into a processor, not shown, through a preamplifier 14. Furthermore, a ring-like electrode 15 is provided around the pressure limiting plate 7. A positive voltage relative to the sample 10 is applied to the electrode 15 by a variable voltage source 16. A secondary electron signal obtained from the electrode 15 is fed into the processor, not shown, through a preamplifier 17.
When the specimen or sample 10 is to be observed, a primary electron beam emitted from the electron gun 3 in the vacuum chamber 2 passes through the aperture 7a in the pressure limiting plate 7 or member and focuses on the sample 10. The sample 10 is scanned by the focused electron beam. This scanning causes secondary electrons to be reflected from the sample 10. The secondary electrons receive energy from the electric field of the pressure limiting plate 7 and/or the electrode 15 and collide with the gas in the sample chamber 9 to ionize the gas. The fact that electrons obtained as a result of this ionization contribute to the magnification of the secondary electrons is gas magnification. On the other hand, the sample 10 is irradiated with positive ions generated as a result of this gas magnification. This irradiation with the primary electron beam neutralizes a negative charge generated in the sample 10. The gas magnified secondary electrons are collected by the electrode 7 serving also as a pressure limiting aperture plate and/or the electrode 15. Thus formed secondary electron signal is fed into the external processor through the preamplifiers 14, 17. Proper use of the two electrodes 15 and 7, is described below. For the observation of a usual sample, the electrode 15 having a longer migration length of secondary electrons is used (in this case, while observing a displayed image of the sample, an operator adjusts a voltage of the variable voltage sources 13, 16 in order to apply a voltage, at which the operator can observe the sample best, to the electrodes 7, 15; of course, a voltage applied to the electrode 7 may be zero in some cases). For an object requiring observation from directly above, the electrode 7 is used (in this case, while observing a displayed image of the sample, an operator adjusts a voltage of the variable voltage sources 13, 16 in order to apply a voltage to the electrodes 7, 15 so that the operator can observe the sample best; of course, a voltage applied to the electrode 15 may be zero in some cases).
The above mentioned conventional construction has a drawback that when the electrode 7 is brought closer to the sample in order to observe a contact hole under optimum conditions, the electrode 7 interferes with detection by the electrode 15. This results in degrading of a sample image, i.e. a poor observation associated with the electrode 15.